It is well-known that there is pressure to eliminate lead compounds from fuels for reasons of public health and environmental protection. Although the specifications for reformulated gasolines set by EPA will come into force in 1995, standards were brought into force on November 1, 1992 requiring gasoline contain 2.7 wt % oxygen during the winter in nonattainment areas of the U.S. If the federal air quality standard for CO has not been achieved by a specified attainment date, the minimum oxygen content will increase to 3.1%. Moreover, starting in the summer of 1992, the maximum blending Reid vapor pressure (BRvp) of all gasolines is set at 9.0 psi. Since oxygenates are not only used as gasoline blending components, extenders, octane boosters and as key ingredients for reducing the emissions of CO and VOCs (Volatile Organic Compounds), it is expected that the demand for oxygenates will increase enormously in the coming years. See F. Cunill, et al., "Effect of Water Presence on Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether Liquid-Phase Synthesis". IND. ENG. CHEM. RES. 1993, 32, 564-569.
Of all oxygenates, the tertiary ethers, such as methyl t-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) are preferred by refineries to lighter alcohols. They have lower blending Ried vapor pressure (BRvp), lower vaporization latent heats and low solubilities in water. The most common ether in use today is MTBE with a production of about 25 million metric tons. However, ETBE is becoming more attractive as the price of methanol goes up in relation to gasoline. It can be produced from renewable ethanol, in contrast to methanol derived from natural gas, and its use would help mitigate the greenhouse effect, Ibid., p. 564.
In addition, ETBE outranks MTBE as an octane enhancer and its BRvp is only 4 psi, which makes it more attractive for BRvp blends less than 8 psi required in some places during the summer. Therefore, a number of U.S. states and European countries are planning to make ETBE from bioethanol, Ibid.
At the present time, TAME, which is usually produced in MTBE refinery units when C.sub.5 olefins are diverted into the feed, is not viewed as rivaling MTBE or ETBE, Ibid.
In an article entitled "C.sub.5 Olefins--The New Refinery Challenge," Kerry Rock et al. discuss the removal of FCC C.sub.5 olefins. See FUEL REFORMULATION, Vol. 2, No. 6, p. 42.
The main drawback of tertiary ethers, is that they substantially increase aldehyde emissions which are under EPA regulations and have to decrease 15% by 1995. It is believed this drawback could be largely circumvented by mixing the tertiary ethers with tertiary alcohols. Tertiary butyl alcohol (tBA) has a very low atmospheric reactivity and low aldehyde emissions, since no hydrogens are contained in the carbon link to the oxygen. Basis experience acquired with tBA during the 1970s, a gasoline blended with a mixture of ethers and tBA and/or tertiary amyl alcohol should be shippable, Ibid.
Generally, it is known that asymmetrical C.sub.4 -C.sub.7 alkyl tertiary alkyl ethers are particularly useful as octane improvers for liquid fuels, especially gasoline. Methyl tertiary butyl ether (MTBE), ethyl t-butyl ether (ETBE), isopropyl t-butyl ether (IPTBE) and tertiary amyl methyl ether (TAME) are known to exhibit high octane properties. Much attention has been focused on production of these ethers due to the rapidly increasing demand for lead-free octane boosters for gasoline.
It is known in the art to produce MTBE or ETBE by reacting isobutylene with either methanol or ethanol, resulting in the formation of MTBE or ETBE, respectively. The reaction normally is conducted in liquid phase with relatively mild conditions. The isobutylene can be obtained from various sources, such as naphtha cracking, catalytic cracking, etc. The resulting reaction product stream contains the desired MTBE or ETBE, as well as unreacted isobutene and other C.sub.4 hydrocarbons and methanol or ethanol.
A number of U.S. patents and allowed U.S. applications assigned to Texaco Chemical Co. disclose methods of making alkyl tertiary alkyl ethers in one step.
In U.S. Pat. No. 4,822,921, to Texaco Chemical Co., there is described a method for preparing alkyl tertiary alkyl ethers which comprises reacting a C.sub.1 -C.sub.6 primary alcohol with a C.sub.4 -C.sub.10 tertiary alcohol over a catalyst comprising an inert support impregnated with phosphoric acid.
U.S. Pat. No. 4,827,048, to Texaco Chemical Co., describes a method for preparing alkyl tertiary alkyl ethers from the same reactants using a heteropoly acid on an inert support.
U.S. Pat. No. 5,099,072, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers over an acidic montmorillonite clay catalyst which possesses very specific physical parameters.
U.S. Pat. No. 5,081,318, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers by reacting a C.sub.1 -C.sub.6 primary alcohol with a C.sub.4 -C.sub.10 tertiary alcohol over a catalyst comprising a fluorosulfonic acid-modified zeolite.
U.S. Pat. No. 5,059,725, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols over a catalyst comprising ammonium sulfate or sulfuric acid on a Group IV oxide.
U.S. Pat. No. 5,157,162, to Texaco Chemical Co., discloses a fluorosulfonic acid-modified clay catalyst for the production of alkyl tertiary alkyl ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols.
In U.S. Pat. No. 5,162,592, to Texaco Chemical Co. there is described a method for producing alkyl tertiary alkyl ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols using a multimetal-modified zeolite catalyst.
A hydrogen fluoride-modified montmorillonite clay catalyst is employed in U.S. Pat. No. 5,157,161, to Texaco Chemical Co., to produce alkyl tertiary alkyl ethers, including ETBE.
In U.S. Pat. No. 5,183,947, to Texaco Chemical Co., fluorophosphoric acid-modified clays are employed as catalysts in a method to produce alkyl tertiary alkyl ethers.
In allowed U.S. Ser. No. 07/917,218, assigned to Texaco Chemical Co., there is disclosed the use of a super acid alumina or a faujasite-type zeolite to produce alkyl tertiary alkyl ethers.
Allowed U.S. Ser. No. 07/878,121, to Texaco Chemical Co., discloses the use of a haloacid-modified montmorillonite clay catalyst to convert C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols to alkyl tertiary alkyl ethers.
Fluorophosphoric acid-modified zeolites are employed in allowed U.S. Ser. No. 07/917,885, to Texaco Chemical Co., to produce alkyl tertiary alkyl ethers.
Other references in the art which use tertiary butanol as a reactant and disclose MTBE or ETBE as a product usually require two stages rather than one.
The use of zeolites for certain reactions is known in the art. Beta-zeolite was first synthesized at Mobil R and D labs and exhibited improved thermal and acid stability over previously synthesized zeolites.
J. B. Higgins, et al. of Mobil Research and Development published an article in ZEOLITES, 1988, Vol. 8, November, 446-452 titled "The Framework Topology of Zeolite Beta." In the article Higgins et al. disclose what is known about the framework topology of .beta.-zeolite. The information was determined using a combination of model building, distance-least-square refinement and powder pattern simulation.
In an article titled "Cumene Disproportionation over Zeolite .beta. I. Comparison of Catalytic Performances and Reaction Mechanisms of Zeolites," APPLIED CATALYSIS, 77 (1991) 199-207, Tseng-Chang Tsai, Chin-Lan Ay and Ikai Wang disclose a study demonstrating that cumene disproportionation can be applied as a probe reaction for zeolite structure. It is revealed that .beta.-zeolite would have application potential in the production of diisopropylbenzene for reasons of activity, selectivity and stability.
In a second part of the article by Tsai et al., "II. Stability Enhancement with Silica Deposition and Steam Pretreatment", Ibid, pp. 209-222, Tsai and Wang disclose their development of two methods to improve the stability of .beta.-zeolite, silica deposition and steam pretreatment.
E. Bourgeat-Lami et al. have published an article discussing their study of the effects of calcination of as synthesized or ammonium-exchanged forms of .beta.-zeolite. See "Stability of the Tetrahedral Aluminum Sites in Zeolite Beta," E. Bourgeat et al., CATALYSIS LETTERS, 1990, 5, p. 265. These researchers came to the conclusion that the tetrahedral aluminum sites disappearing upon calcination can be readily restored by a simple treatment in ammonium nitrate. The parent sample of .beta.-zeolite with a Si/Al ratio of 16.9 was synthesized at 130.degree. C. using tetraethylammonium hydroxide as template. The NMR spectrum indicated a dealumination corresponding to about 25%. When this material was treated with ammonium nitrate solution, washed and over dried at 70.degree. C., the signal of octahedral aluminum was no longer detected while that at 53 ppm narrowed and increased to 95% of its original value.
Patents in the art which employ .beta.-zeolite relate mainly to dewaxing, and cracking of hydrocarbon feedstock.
An article titled "Beta Zeolite as Catalyst or Catalyst Additive for the Production of Olefins During Cracking of Gas Oil," was written by L. Bonetto et al., 9th International Zeolite Conference, July 1992, FP 22. The authors note that with the greater demand for oxygenated compounds there is indication there might be increased demands for catalysts and conditions which maximize C.sub.3, C.sub.4 and C.sub.5 olefins. They suggest that .beta.-zeolite could be used alone or combined with Y-zeolite as a suitable zeolite component. Various catalysts were studied with respect to minimization of diffusional requirements and zeolite stability.
U.S. Pat. No. 4,419,220, to Mobil, discloses a process for dewaxing a hydrocarbon feedstock containing straight chain paraffins which comprises contacting the feedstock with a .beta.-zeolite catalyst having a Si:Al ratio of at least 30:1 and a hydrogenation component under isomerization conditions.
Another European Application to Mobil, EPO 0 094 827, discloses simultaneous catalytic hydrocracking and hydrodewaxing of hydrocarbon oils with .beta.-zeolite.
In European Patent Application 0 095 303, to Mobil, there is a disclosure of dewaxing distillate fuel oils by the use of .beta.-zeolite catalysts which, preferably have a silica:alumina ratio over 100:1. Ratios as high as 250:1 and 500:1 are disclosed as useful.
Another U.S. Pat. 4,518,485, to Mobil, discloses a process for dewaxing a hydrocarbon feedstock containing paraffins selected from the group of normal paraffins and slightly branched paraffins and sulfur and nitrogen compounds where, after conventionally hydrotreating the feedstock to remove sulfur and nitrogen, the hydrotreated feedstock is dewaxed by contacting the feedstock with a catalyst comprising .beta.-zeolite having a silica/alumina ratio of at least 30:1.
In U.S. Pat. No. 4,740,292, to Mobil, there is disclosed a catalytic cracking process which comprises cracking a hydrocarbon feed in the absence of added hydrogen with a cracking catalyst comprising a .beta.-zeolite component and a faujasite component comprising at least one crystalline aluminosilicate of the faujasite structure, the weight ratio of the faujasite component to the .beta.-zeolite component being from 1:25 to 20:1.
Large pore .beta.-zeolite has been employed in the synthesis of industrially important para-cumene by toluene isopropylation. See "Toluene Isopropylation over Zeolite .beta. and Metallosilicates of MFI Structure," P. A. Parikh et al., APPLIED CATALYSIS, A, 1992, 90, p. 1.
It would be a valuable advance in the art if a method were available for the synthesis of methyl and ethyl tertiary butyl ether from tertiary butanol employing one step technology and a catalyst exhibiting product phase separation at operating temperatures of 140.degree. C. or higher. The cosynthesis of isobutylene and diisobutylene using the crude feedstock reactants would make such a process even more attractive.